The present invention relates to a device for the on-the-move heating, by electromagnetic induction, of magnetic or amagnetic strips of small and medium thicknesses (of the order of 0.05 to 50 millimetres). It is more particularly aimed at a transverse flux induction heating device.
In a known manner, the on-the-move heating by electromagnetic induction of a metal strip is carried out with the aid of coils which are arranged in such a way as to surround the strip to be heated while creating a magnetic field parallel to the outer surface of this strip in the direction of travel (longitudinal flux, cf. FIG. 1a). A ring-like distribution is thus obtained of the induced currents which traverse the continuously moving strip in the vicinity of its peripheral surface, this giving rise to heating whose transverse temperature homogeneity is generally regarded as satisfactory.
When dealing with the heating of magnetic strips of small thickness, the efficiency of this type of heating with longitudinal flux is high. However, it drops steeply, for these materials, as soon as the Curie point temperature (around 750xc2x0 C.) is exceeded. This is due in particular to the fact that the relative permeability of the material to be heated decreases rapidly during the heating process, reaching the value 1 at this same temperature. The efficiency is also limited for amagnetic materials (stainless steel, aluminium, etc.), regardless of the temperature of the product.
According to another known solution for the on-the-move induction heating of flat metal products, two coils are arranged on either side of the product to be heated up, opposite each of the large faces thereof so as to create a magnetic field perpendicular to the large faces of the product according to the so-called transverse flux technique (cf. FIG. 1b).
The main drawback of this type of plant lies in the fact that the looped distribution of the currents induced by the crosswise magnetic flux does not generally allow a satisfactory temperature homogeneity to be achieved, in particular the ends in the width direction of the strip (the edges) are heated excessively or insufficiently depending on the relative dimensions of the coils and of the magnetic circuit which are used as compared with the strip width.
To solve this problem, the use has already been proposed of transverse flux electromagnetic induction heating in which the inductors comprise magnetic circuits. The latter are intended to guide the magnetic flux generated by the coils so as to act on the distribution of the induced currents.
However, such devices have the disadvantage of not being easily modifiable so as to adapt to the widths of strip to be heated. To counter such a drawback, there is known for example an electromagnetic induction heating device described in American Patent No. 4,678,883 in which the inductors consist of a plurality of mutually coupled magnetic bars (the term xe2x80x9ccoupledxe2x80x9d is understood to mean bars which co-operate with one another such that the magnetic flux produced by the inductors can pass from one bar to the other bar), which are arranged parallel to the direction of movement of the strip to be heated and can be individually moved perpendicularly to the surface of the said strip in such a way as to adapt the flux distribution to the width of the strip, according to the latter""s dimensions.
Now, even this type of electromagnetic induction heating does not allow correct control of the temperature fluctuations in the vicinity of the edges of the strip to be heated. Specifically, the magnetic bars set back with respect to the said strip continue to exert an influence, albeit weaker, on the magnetic flux distribution and hence on the temperature and as a result of this the temperature distribution curve shows a concentration of the currents induced on the edges.
Moreover, likewise known is EP-A-0 667 731 which discloses a transverse flux electromagnetic induction heating device in which the length of the coils is varied so as to adapt the flux distribution to the strip widths. To do this, this document proposes that these coils be made by assembling two J-shaped opposed inductors which can translate freely in a direction parallel to the strip width. As in the American patent mentioned above, this device does not make it possible to obtain very satisfactory transverse temperature homogeneity.
In view of the drawbacks of the solutions of the prior art recalled hereinabove, the present invention proposes to provide an original solution by making a transverse flux electromagnetic induction heating device whose magnetic circuit, made with a plurality of independent magnetic bars, adapts to the width of the strip to be heated. This device thus makes it possible to improve the thermal homogeneity in the width direction of the strip to be heated.
Accordingly, the invention provides a device for the electromagnetic induction heating of a metal strip travelling in a specified direction comprising at least one electric coil arranged opposite at least one of the large faces of the said strip so as to heat the latter by transverse magnetic flux induction, each coil being associated with at least one magnetic circuit, each circuit being divided into a plurality of mutually uncoupled magnetic bars arranged parallel to the direction of travel of the strip, the said device being characterized in that the said magnetic circuit, consisting of the said plurality of mutually independent bars, adapts to the width of the strip to be heated by moving the said bars away from or towards one another, in such a way as to continuously adapt the distribution of the said magnetic flux to the characteristic dimensions of the said strip.
Thus, by virtue of the present invention, regardless of the width of the strip to be heated, the volume and hence the weight of the magnetic circuit remains invariable.
According to an advantageous characteristic of the invention, the electromagnetic induction heating device also comprises screens made of materials of good electrical conductivity placed in the gap on either side of the strip and in the vicinity of the latter""s edges, in such a way as to optimize the homogeneity of the transverse temperature.
According to another advantageous characteristic of the invention, the surface of the magnetic circuit which is opposite one of the large faces of the strip to be heated is given a suitable xe2x80x9cpolarxe2x80x9d profile (bisinusoidal for example) by fashioning the magnetic laminations constituting this circuit such as to obtain a better distribution of the magnetic flux, and more especially in the vicinity of the edges of the said strip. The term xe2x80x9cpolarxe2x80x9d profile is understood to mean a surface of the magnetic circuit which is curved in the three directions in space.
Other characteristics and advantages of the present invention will emerge from the description given hereinafter, with reference to the appended drawings which illustrate exemplary embodiments and applications thereof, devoid of any limiting character.